Vapor recovery fuel dispensers, particularly gasoline dispensers, have been known for quite some time, and have been mandatory in California for a number of years. The primary purpose of using a vapor recovery fuel dispenser is to retrieve or recover the vapors which would otherwise be emitted to the atmosphere during a fueling operation, particularly for motor vehicles. The vapors of concern are generally those which are contained in the vehicle gas tank. As the liquid gasoline is pumped into the tank, the vapor is displaced and forced out through the filler pipe. Other volatile liquids such as hydrocarbon fluids raise similar issues.
The traditional vapor recovery apparatus is known as the “balance” system, in which a sheath or boot encircles the liquid fueling spout and connects with tubing back to the fuel reservoir. As the liquid enters the tank, the vapor is forced into the sheath and back toward the fuel reservoir where the vapors can be stored or recondensed.
Balance systems have numerous drawbacks, including cumbersomeness, difficulty of use, ineffectiveness when seals are poorly made, and slowed fueling rates.
As a dramatic step to improve on the balance systems, Gilbarco, Inc., assignee of the present invention, patented an improved vapor recovery system for fuel dispensers, U.S. Pat. No. 5,040,577 to Kenneth L Pope. The Pope patent discloses a vapor recovery apparatus in which a vapor pump is introduced in the vapor return line, driven by a motor. The liquid flow line includes a pulser, conventionally used for generating pulses indicative of the amount of liquid fuel being pumped. This permits computation of the total sale and the display of the volume of liquid and the cost in a conventional display, such as, for example as shown in U.S. Pat. No. 4,122,524 to McCrory et al. A microprocessor translates the pulses indicative of the liquid flow rate into a desired vapor pump operating rate. The effect was to permit the vapor to be pumped at a rate correlated with the liquid flow rate so that, as liquid is pumped faster, vapor is also pumped faster.
While the apparatus described in detail in the Pope patent is significant and quite workable, various improvements and refinements have been discovered to further enhance the usability of it and similar vapor recovery systems.
In particular, since the vapor pump is independently driven, in the event of a malfunction so that the vapor pump is operating when the liquid pump is not, there is a possibility of drawing large volumes of air into the liquid storage tank. When the quantity of air reaches a high enough level, the air/vapor mixture in the tank can reach dangerously explosive proportions. Accordingly, safety features are needed to assure that excessive amounts of air are not drawn in.
Further, it has been found that if liquid is pumped back through the vapor pump line in large quantities, damage to the vapor pump can result, so that a need is present to deal with that circumstance.
In dispensing systems for vaporizable liquid, the liquid flows to the tank being filled through a tube and vapor is sucked by a recovery pump from the tank via an adjacent tube. If the temperature of the liquid and the temperature of the vapor in the tank are the same, the volumetric flow VR of the vapor recovery pump can be made more or less equal to the volumetric flow of the liquid, VL. However, if the temperatures are different, a heat exchange takes place between the liquid and the vapor in the vehicle tank so that the vapor expands or contracts in accordance with the universal gas law PV=mRT. Therefore, in order to evacuate all of the vapor that is displaced from the tank as the liquid enters it and yet not suck in excess air by sucking too hard, the volumetric flow of the vapor recovery pump must be varied. By way of example, if the temperature of the vapor in the tank being filled is colder than the liquid being pumped into it from an underground reservoir, as may well occur during winter, the vapor in the vehicle tank will be heated and will expand, thereby requiring an increase in the volumetric flow of the vapor pump. The opposite effects may take place during the summer. Compensation of the vapor flow rate to account for these differences is needed.